1. Field of the Invention
The present invention relates to an apparatus and method for controlling the speed of a 3-phase motor using four switching elements, and more particularly to a phase distortion compensating apparatus and method for reducing a torque ripple in a 3-phase motor using four switching elements, which are capable of directly or indirectly detecting a voltage difference between upper and lower DC link capacitors connected to respective switch legs of an inverter including the switching elements, and adjusting respective switching times of phase voltages, based on the detected voltage difference to reduce a torque ripple generated in the 3-phase motor.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating a conventional 3-phase motor controller using four switching elements. As shown in FIG. 1, the 3-phase motor controller includes a pair of DC link capacitors, that is, an upper DC link capacitor 3 and a lower DC link capacitor 4, respectively adapted to receive a DC voltage rectified from an AC voltage, and to store the DC voltage therein. The 3-phase motor controller also includes a B4 inverter 2 configured to turn on or off in response to a switch control signal when the DC voltage from each of the capacitors 3 and 4 is supplied, thereby supplying a 3-phase voltage adapted to rotate a 3-phase motor 1. The 3-phase motor 1 is coupled to respective switch legs of the B4 inverter 2 and coupled to a connection node between the upper and lower DC link capacitors 3 and 4.
Generally, inverters are known which use six switching elements to control a 3-phase motor. However, such inverters are expensive. In order to reduce the costs of such inverters, an inverter has been proposed which uses four switching elements to control a 3-phase motor. An example of such an inverter is the B4 inverter 2 shown in FIG. 1.
Now, the operation of the conventional 3-phase motor controller using four switching elements as mentioned above to control the 3-phase motor will be described.
When an AC voltage is inputted to the 3-phase motor controller, it is rectified by a rectifier means (not shown) which, in turn, generates a DC voltage. This DC voltage is supplied to the upper and lower DC link capacitors 3 and 4 connected in parallel to each other.
As a result, the upper and lower DC link capacitors 3 and 4 conduct charge and discharge operations in an alternating fashion. The alternating charge and discharge operations are controlled in accordance with respective status changes of switching elements composing the B4 inverter 2.
The B4 inverter 2 has four switching status, as shown in FIGS. 3a to 3d. The following description will be made in association with the case in which the 3-phase motor has a Y-connection. In the following description, xe2x80x9c1xe2x80x9d means an ON state of the upper switching elements in the B4 inverter 2 whereas xe2x80x9c0xe2x80x9d means an ON state of the lower switching elements. Where only the upper ones of the switching elements respectively corresponding to four voltage vectors of the B4 inverters are switched on, that is, in a status  less than 1, 1 greater than , the voltage V1 charged in the upper DC link capacitor 3 is supplied to the 3-phase motor 1. In this status, no voltage is supplied from the lower DC link capacitor 4 to the 3-phase motor 1.
On the other hand, when only the lower switching elements are switched on, that is, in a status  less than 0, 0 greater than , the voltage V2 charged in the lower DC link capacitor 4 is supplied to the 3-phase motor 1. In this status, no voltage is supplied from the upper DC link capacitor 3 to the 3-phase motor 1.
In statuses  less than 0, 1 greater than and  less than 1, 0 greater than , both the upper and lower DC link capacitors 3 and 4 supply the voltages V1 and V2 to the 3-phase motor 1, respectively.
In order to allow the 3-phase motor 1 to rotate, it is necessary to generate voltages of three phases each exhibiting a phase difference of 120xc2x0 from one another, Va, Vb and Vc, as shown in FIG. 2.
In order to generate these voltages of three phases, one of three nodes in the B4 inverter 2 respectively corresponding to voltage vectors of three phases applied to the 3-phase motor 1 is connected to the connection node between the upper and lower DC link capacitors 3 and 4, and the remaining two nodes are connected to respective legs between the upper switching elements and the associated lower switching elements.
Also, all 3-phase voltage vectors have not direction, and conclusively xe2x80x9cVuxe2x80x9d and xe2x80x9cVwxe2x80x9d is only generated, as depicted FIG. 2 when a voltage-Vc is applied to the voltage Va, Vb, Vc in order to generate voltage having the same effect as balanced 3-phase voltages.
The voltage vectors Vu and Vw serve to generate balanced 3-phase voltages along with a voltage of zero-phase. That is, it is possible to obtain 3-phase balanced voltages using four switches.
Two voltage vectors Vu and Vw generated by the B4 inverter 2 have a phase difference of 60xc2x0 therebetween, as shown in FIG. 2. In the case in which the c-phase of the 3-phase motor is connected to the connection node between the upper and lower DC link capacitors 3 and 4, as mentioned above, the phase of the voltage vector Vu is retarded from the a-phase voltage Va by 30xc2x0.
Therefore, where the B4 inverter 2 is controlled using pulse width modulated (PWM) pulses, it is possible to control the 3-phase motor 1 using a switching logic of the B4 inverter 2 expressed by the following Equation 1:                                           V            u                    =                                    V              a_dc                        =                                          [                                                      1                    2                                    +                                                            1                      2                                        ·                    ma                    ·                                          sin                      ⁡                                              (                                                  θ                          -                                                      π                            6                                                                          )                                                                                            ]                            ·                              T                samp                                                    ⁢                  
                ⁢                              V            w                    =                                    V              b_dc                        =                                          [                                                      1                    2                                    +                                                            1                      2                                        ·                    ma                    ·                                          sin                      ⁡                                              (                                                  θ                          -                                                      π                            2                                                                          )                                                                                            ]                            ·                              T                samp                                                                        (        1        )            
where, xe2x80x9cxcex8xe2x80x9d represents a rotor position, xe2x80x9cmaxe2x80x9d represents a modulation rate, and xe2x80x9cTsampxe2x80x9d represents a switching sampling time.
The above Equation 1 is associated with the case in which the c-phase of the 3-phase motor is connected to the connection node between the upper and lower DC link capacitors. Referring to Equation 1, it can be found that the voltages Vw and Vu have a phase difference of 60xc2x0 therebetween, and the voltage Vu is retarded in phase from the voltage Va by 30xc2x0.
In accordance with the above mentioned conventional method, the supply of a voltage to the motor has two statuses, that is, a status, in which the voltage is supplied based on only one of the upper and lower DC link capacitors is used, and a status, in which the voltage is supplied based on both the upper and lower DC link capacitors are used, in accordance with the switching statuses of the switching elements composing the B4 inverter. For this reason, there is a voltage difference between the upper and lower DC link capacitors, so that it is impossible to apply a balanced 3-phase voltage to the 3-phase motor. As a result, a torque ripple occurs. Due to such a torque ripple, it is impossible to achieve a reliable speed control.
The present invention has been made in view of the above mentioned problems involved in the related art, and an object of the invention is to provide a phase distortion compensating apparatus and method for reducing a torque ripple in a 3-phase motor using four switching elements, which are capable of adjusting respective switching times of phase voltages, to be supplied to the 3-phase motor by an inverter including the switching elements, based on a voltage difference between upper and lower DC link capacitors respectively adapted to supply voltages to the inverter, thereby reducing a torque ripple generated in the 3-phase motor.
Another object of the invention is to provide a phase distortion compensating apparatus and method for reducing a torque ripple in a 3-phase motor using four switching elements, in which a torque ripple generated in the 3-phase motor is reduced, based on a voltage difference between upper and lower DC link capacitors supplied to an inverter including the switching elements and detected using a current flowing through the 3-phase motor.
In accordance with one aspect, the present invention provides a phase distortion compensating apparatus for reducing a torque ripple in a 3-phase motor, comprising: a rectifier unit for rectifying an input AC voltage into a DC voltage; upper and lower DC link capacitors connected in parallel to the rectifier unit, each of the DC link capacitors serving to conduct charge and discharge operations for the DC voltage; an inverter connected in parallel to the capacitors and adapted to generate a 3-phase voltage adapted to rotate the 3-phase motor, based on voltages respectively discharged from the capacitors along with a switching signal; and a voltage command generator for calculating compensation components for respective switching operations of switching elements of A and B phases included in the inverter, based on a voltage difference between the capacitors, a difference between an actual rotating speed of the motor and a command speed, and a rotor position of the motor, and providing respective switching times including the calculated compensation components, thereby controlling a rotating speed of the motor.
In accordance with another aspect, the present invention provides a phase distortion compensating method for reducing a torque ripple in a 3-phase motor, comprising the steps of: (a) calculating respective switching times (Ta and Tb) for upper ones of four switching elements respectively associated with A and B-phase legs in an inverter, based on a voltage command for driving the 3-phase motor; (b) comparing the calculated switching times (Ta and Tb) with each other; (c) calculating on-time t1 of lower switch in the A and B-phase leg switching status from the Ta and Tb and on-time t2 of upper switch in the A and B-phase leg switching status from the Ta and Tb, (d) detecting a current inputted to a connection node between upper and lower DC link capacitors respectively connected to the A and B-phase legs of the inverter after completion of the calculation for the switching times (t1 and t3), and integrating the detected current, thereby deriving a difference between a voltage across the upper DC link capacitor and a voltage across the lower DC link capacitor, (e) calculating respective compensation components (xcex94Ta and xcex94Tb) for the switching times (Ta and Tb), based on the derived voltage difference, and (f) producing new switching times (Taxe2x80x2 and Tbxe2x80x2) respectively reflecting the calculated compensation components, and supplying the new switching times to the inverter.